1. Field of the Invention
This invention relates to non-volatile semiconductor memory and to methods for maintaining accurate storage of data in a non-volatile semiconductor memory.
2. Description of Related Art
A conventional non-volatile memory such as a Flash memory has memory cells that include floating gate transistors. Each floating gate transistor stores data as charge trapped on an isolated floating gate. The charge trapped on the floating gate transistor determines the threshold voltage of the floating gate transistor, and a data value can be written to or read from a memory cell by setting or determining the threshold voltage of the floating gate transistor in the memory cell. If the methods for setting and determining threshold voltages are sufficiently accurate and dependable, multiple bits of data can be stored in each memory cell.
Time-dependent and voltage-dependent charge leakage from or onto the floating gate of a memory cell can change the threshold voltage of the memory cell. Voltage-dependent usually leakage results from programming or other operations on neighboring memory cells. Time-dependent charge leakage generally depends on the temperature of a memory circuit but occurs even when the memory is not accessing (i.e., erasing, programming, or reading) any memory cells. Conventionally, retention of stored data requires minimizing the charge leakage to maintain the threshold voltages of memory cells that store data and/or tracking changes in the threshold voltages.
Tracking circuits can use reference cells that are subject to charge leakage that is similar to memory cells storing data, and a comparison of a memory cell and a reference cell can indicate a correct data value even if the threshold voltages have changed. Such tracking circuits can also track changes in measured threshold voltages that result from differences in operating parameters such as the supply voltage or temperature for the memory. However, tracking circuits cannot track changes in threshold voltage for every memory cell with complete accuracy. Accordingly, each data value corresponds to a range of threshold voltages, and that range must be made sufficiently wide to cover variations in the threshold voltages representing the same data value. Since the full usable range of threshold voltages of a memory cell is limited, having a wide range for each data value reduces the number of bits that can be stored per memory cell. Accordingly, methods and circuits are sought for maintaining threshold voltages within narrow windows to ensure data integrity over time.